PCB Reflow with a Skillet: Part 1

The skillet I bought is a Farberware 12" x 12" ceramic skillet, model FW6110027877. Searching the web for this model turns up nothing, so it's probably specific to Wallyworld. This idea is supported by the absurdly short power cord, which is barely 2 feet long including the plug. I did my test with the skillet under the vent hood above my stove, which meant I had to use a heavy-duty extension cord to reach the nearest outlet in the wall just to the side of the stove. Annoying shortcuts like this is how they sell at such low prices.

My initial plan was set the skillet's control to its maximum temperature of 450℉ (232℃) and record the surface temperature of the empty skillet in the center and the corners of an imaginary 9" square as it warmed up. I'd been thinking the center of the skillet would be its hottest spot, but it's actually one of the coolest. Within two minutes the center had topped 350℉ (180℃) and the corners hotter, too fast for me to record good readings, so I gave up trying to be scientific about it. By three minutes the center had reached 450℉ (230℃) and was still rising, while the corners were approaching 500℉ (260℃). Clearly hot enough to reach my target temperatures.

A look at the underside helps explain this odd temperature distribution. The heating element on the underside of the skillet is circular, which put the corners of my imaginary 9" square much closer to the element than the center.

As a test I'd stenciled some solder onto an extra Instruction Decoder board without any components. I slipped the board into the hot skillet and watched my thermometer as the board temperature rose rapidly. The uneven heating was obvious, as the solder in some areas of the board melted well before others, but within just a couple of minutes the solder on all areas had melted. I turned the skillet off and watched as the board cooled to the point the solder solidified, then scooped the board out with a thin spatula and put it on a rack to cool enough that I could handle it. There were no visible changes or scorch marks on either side of the board to show it'd been heated.

Sn63/Pb37 reflow profile

Sliding a cold board into a hot skillet isn't something I'd do with a populated board. The standard solder reflow profile specifies that the temperature should rise in stages. The first stage, preheat, brings the board and components fairly quickly to about 100-150℃. The second stage, thermal soak, slowly increases the temperature over about a 120 second period to near the melting point of the solder. This slow ramp minimizes the thermal stresses within the components and the board, activates the flux, and allows any moisture or volatiles to evaporate without turning to steam and causing damage. The third phase, reflow, sees the temperature rise to its peak for about 60 seconds, and is when the solder actually melts and bonds to the component leads and PCB pads. The fourth and final phase, cooling, sees the temperature fall to form good solder joints.

I can't hope to match the reflow profiles used for commercial production. I'll try to approximate it by tweaking the control while monitoring the board temperature with my infrared thermometer, but if everything reflows without damaging anything I'll call it good enough. I don't mind touching up a few joints here and there.

Something I noticed while doing this test is that the boards I received are not completely flat. They're not as warped as the ID board after I assembled it, but they're they rock on two corners when placed on a flat surface. Nor is the surface of the skillet flat, with a high spot in the center. A suggestion I read was to put a thin layer of sand between the skillet and the board to provide good conductive heat transfer and perhaps even out the uneven heating. This would slow down the thermal ramps too, though I don't know if that's good or bad in this case.